1. Field of the Invention
The present invention relates to an optical communication network, and more particularly to a subcarrier-type passive optical network.
2. Description of the Related Art
Optical networks are being more widespread, as they are suitable for various multimedia services based on extensive use of Internet and broadband signal transmission. The optical networks are typically used in-line with FTTx (Fiber To The x) technology for guaranteeing Gigabit per second (Gbps)-grade transmission speed by using optical fibers.
Optical communication methods applicable to optical networks include (1) a wavelength division multiplexing method, in which a plurality of different wavelengths are allocated to respective subscribers so that light can be allocated based on the wavelength, (2) a time division multiplexing method, in which a single wavelength of light is split into a plurality of time slots, and the time slots are allocated to respective subscribers; and (3) a subcarrier multiplexing method, in which a single wavelength of light is split into a plurality of channels, and the channels are loaded with different proper subcarrier frequencies that are allocated to respective subscribers.
According to the subcarrier multiplexing method, a single wavelength is shared by a number of subscribers, in a manner similar to that of the time division multiplexing method. The subcarrier multiplexing method is applicable to a passive optical network, for example, which includes a central office, a plurality of subscribers, and a remote node positioned between the central office and the subscribers.
Base on transmission requirements, one of the wavelength division multiplexing method, the time division multiplexing method, or the subcarrier multiplexing method may be selected and used for optical networks.
FIG. 1 shows the brief construction of a conventional passive optical network. Referring to FIG. 1, the conventional passive optical network 100 includes an optical line terminal 110, a remote node 120, and a plurality of optical network units 130-1 to 130-N.
The optical line terminal 110 includes a light source 112 for creating downstream (downward) optical signals to be provided to the optical network units 130-1 to 130-N, an upstream receiver 113 for detecting data from upstream optical signals received from the respective optical network units 130-1 to 130-N, and a wavelength division multiplexer 111.
FIG. 2a is a graph showing downstream optical signals, which are obtained by multiplexing a plurality of subcarrier channels. The light source 112 creates downstream optical signals 101 (λD) consisting of subcarrier channels f1-fm, which have proper frequencies allocated to respective optical network units 130-1 to 130-N. The light source 112 may include a frequency creation unit, a modulator, and light creation means.
The remote node 120 is linked to the optical line terminal 110 via a single optical fiber. The remote node 120 may be positioned closest to the optical network units 130-1 to 130-N. The remote node 120 may have an optical splitter 121 having 1×N port structure. As shown in FIG. 2b, the remote node 120 multiplexes upstream (upward) subcarrier channels, which have been created by respective optical network units 130-1 to 130-N, into upstream optical signals and outputs them to the optical line terminal 110.
Each of the optical network units 130-1 to 130-N includes a downstream receiver 132, an upstream light source 133 for creating upstream upstreamsubcarrier channels, and a wavelength division multiplexer 313 linked to the remote node 120 while being connected to the upstream upstreamlight source 133 and the downstream receiver 132. The downstream receiver 132 filters the downstream optical signals so as to obtain a subcarrier channel (one of f1-fm), which has a corresponding frequency, and detects data from it.
The upstream light source 133 creates an upstream subcarrier channel (one of f1-fm), which has a proper frequency, as shown in FIG. 2b. 
However, when upstream subcarrier channels having the same wavelength are detected by a single upstream receiver, interference between them may lead to optical interference. In order to reduce the optical interference, the number of subscribers accommodated by an optical network must be limited. In other words, the optical \interference occurs when subcarriers having different proper frequencies are allocated to channels having the same wavelength and are inputted to a single optical detector. The resulting interference between subcarrier channels creates subcarrier frequencies. Removing this interference increases the cost for installing and maintaining the network.